Emission measure distribution for an active region using coordinated SERTSand Yohkoh SXT observations

Often the derived temperature of an active region reflects the method and t he nature of the instrument used in its measurement. The emission measure ( i.e., the amount of emitting material) derived from spectroscopic observati ons usually depends on assumptions about the absolute elemental abundances and ionization fractions of the emitting ions. Yet establishing the distrib ution of emission measure with temperature is the first step needed to proc eed with most of the interesting physics of active regions-including heatin g processes, cooling timescales, and loop stability. Accurately characteriz ing the thermal distribution of the coronal plasma requires data which can resolve multithermal features and constrain both low- and high-temperature emission. To model the temperature distribution of NOAA Active Region 7563, we have combined broadband filter data from the Yohkoh Soft X-Ray Telescop e (SXT) with simultaneous spectral line data from the Goddard Solar EUV Roc ket Telescope and Spectrograph (SERTS) taken during its flight on 1993 Augu st 17. We have used a forward-folding technique to determine the emission m easure distribution of the active region loops. We have found that (1) the SXT response functions are sensitive to both the elemental abundances and t he ionization fractions assumed to compute the solar spectrum that is folde d through the instrument effective area; (2) the relative calibration betwe en the SERTS and the SXT instruments must be adjusted by a factor of 2 (a v alue consistent with the absolute measurement uncertainty of the 1993 SERTS flight) no matter which abundances or iron ionization fractions are used; (3) the two-peaked differential emission measure previously determined usin g SERTS data alone is not consistent with the SXT data: including the SXT d ata as a high-temperature constraint in the analysis requires that the emis sion above about 3 MK drop off steeply rather than extending out to 6 MK. T he sensitivity of the SXT filter response functions to elemental abundance and iron ionization fraction could have a major impact on many routine anal yses of SXT data. The emission measures can be greatly affected (up to a fa ctor of 7) and temperatures derived from filter ratios can be significantly altered (up to at least 40%) by adopting different sets of commonly used e lemental and ionic abundances. The results of our multithermal analysis imp ly that using broadband SXT data or a comparable high-temperature constrain t in conjunction with high-resolution spectra covering a wide lower tempera ture range to study solar active regions can significantly improve the info rmation derived from either data set alone. In this study, the revised mult ithermal distribution reduces the thermal energy content of the region by a bout a factor of 2 and the required heating by about a factor of 5, which i n turn relaxes some constraints on possible heating models.